This research was sponsored by the National Science Foundation Grant NSF PHY-931478

Ellison, T.

Graham, W.G.

Haar, R.R.

Jacobs, W.W.

Mowat, J.R.

Rinckel, T.

Tanis, J.A.

English

IUScholarWorks

ATOMIC PHYSICS 
DIELECTRONIC RECOMBINATION IN Li+ IONS 
J.A. Tanis and R.R. Haar 
W e s t e r n  Mich igan  Univers i ty ,  Kalamazoo ,  Mich igan  4 9008  
W.G. Graham 
Queen's Univers i ty ,  Bel fast ,  N o r t h e r n  Ireland 
J.R. Mowat 
N o r t h  Carol ina S t a t e  Univers i ty ,  Raleigh, N o r t h  Carol ina  27695  
T. Ellison, W.W. Jacobs, and T. Rinckel 
I n d i a n a  Un ive r s i t y  Cyc lo t ron  Facili ty,  B looming ton ,  I n d i a n a  4 7 4 0 8  
Dielectronic recombination' (DR) is an atomic process occurring in electron-ion col- 
lisions when the capture of an electron is accompanied by the simultaneous electronic 
excitation of the ion resulting in the formation of a doubly-excited intermediate state; 
subsequent deexcitation by photon emission completes the recombination of the ion. The 
DR process, which is mediated by the electron-electron interaction, is just the inverse 
of an Auger transition and is resonant for relative velocities corresponding to outgoing 
Auger-electron energies. 
Over a broad energy range DR is the principal means by which free electrons recom- 
bine with ions. So DR has been of fundamental interest for many years.2j3 In addition, DR 
is important to astrophysical studies4 and to the development of nuclear fusion plasmas;5 
and DR can be an important diagnostic tool for ion-beam cooling in storage rings. Cross 
sections for DR have been calculated2j3 extensively. Recently, powerful new experimental 
methods have provided good rneas~rements~- '~ of DR for testing the theoretical predic- 
t ions. 
In this work, DR was investigated for Li+(ls2) + e- -+ [Lio(lsnln't')]** using the 
electron cooler at IUCF. The present work is an extension of our previous work1'-l2 at 
IUCF for He+ ions. These light He+ and Li+ ions pose stringent tests of DR theory 
because the electron-electron interaction, which mediates DR, is stronger compared to 
electron-nucleus interactions than it is for heavier ions. Additionally, the electron coupling 
in two-electron ~ i +  gives rise to angular momentum configurations different from those in 
HeS. 
For Li+(ls2) ions, DR is expected to occur for relative energies EreI of 50-60 eV 
between the ion and the electron. However, the metastable beam component Li+(ls2s) 
can give rise to DR involving 2s -+ 2p transitions for relative energies near zero (i.e., a few 
eV). Also, for Erel = 0, a peak due to radiative recombination (RR) (inverse photoelectric 
effect) is expected. 
0 
2000 2500 3000 3500 
CHANNEL 
Figure 1. Measured intensity (counts) of neutral Li atoms resulting from collisions of 
31 MeV Li+ ions with electrons in the Cooler region. The channel number is proportional 
to the laboratory electron energy. The expected regions for dielectronic recombination 
(DR) and radiative recombination (RR) are indicated. 
As for ~ e + ,  the ~ i +  measurements were conducted in a "single-pass" mode in which 
the ions circled the storage ring, passed through the intense electron beam in the cooler 
region, and then those Li+ ions not undergoing recombination were deflected by a ring 
dipole magnet and collected in a Faraday cup. Events resulting in DR were detected from 
the yield of neutral Li atoms formed in the cooler region; these atoms emerged through the 
0" exit port following the electron cooler and were counted with a large area (2000 mm2) 
silicon surface-barrier detector. For the measurements conducted here, a 31-MeV beam of 
~ i +  ions of current 10 nA was merged with the electron cooling beam of current 300 mA 
over the interaction length of 2.8 m. 
In these measurements we did observe DR for ~ i + ,  but the beam intensity was too 
low and the beam stability too erratic to give good results. A typical spectrum obtained 
is shown in Fig. 1. Some evidence of the expected DR peak due to ls2 + e- -+ ls2l2l 
transitions in the range 50-60 eV is seen, but the data are not convincing. Also there 
is an indication of a DR contribution resulting from ls2s + e- -+ ls2pnl transitions in 
the metastable ~ i + ( l s 2 s )  ion. However, this latter contribution to DR occurs near zero 
relative energy where radiative recombination (RR) should be at a maximum. In the 
present work, the separate contributions from DR and RR cannot be clearly identified. 
Additional measurements with better beam conditions are planned. 
1. H.S.W. Massey and D.R. Bates, Rep. Prog. Phys. 9 ,  62 (1942). 
2. See Y. Hahn and K.J. La Gattuta, Phys. Rep. 166, 196 (1988), and references therein. 
3.'See Recombination of Atomic Ions, eds. W.G. Graham, W. Fritsch, Y. Hahn, and J.A. 
Tanis (Plenum Press, New York and London, 1992). 
4. A. Burgess, Astrophys. J .  139, 776 (1964); and 141, 1588 (1965). 
5. A.L. Merts, R.D. Cowan, and N.H. Magee, Jr., Los Alamos Sci. Lab. Rep. No. LA- 
6220-MS, 1976. 
6. D.A. Knapp, R.E. Marrs, M.A. Levine, C.L. Bennett, M.H. Chen, J.R. Henderson, 
M.B. Schneider, and J.H. Scofield, Phys. Rev. Lett. 62, 2104 (1989). 
7. L.H. Anderson, P. Hvelplund, H. Knudsen, and P. Kvistgaard, Phys. Rev. Lett. 62, 
2656 (1989). 
8. G. Kilgus, J. Berger, P. Blatt, M. Grieser, D. Habs, B. Hochadel, E. Jaeschke, D. 
Kramer, R. Neumann, G. Neureither, W. Ott, D. Schwalm, M. Steck, R. Stokstad, 
E. Szmola, A. Wolf, R. Schuch, A. Muller, and M. Wagner, Phys. Rev. Lett. 64, 737 
(1990). 
9. R. Ali, C.P. Bhalla, C.L. Cocke, and M. Stockli, Phys. Rev. Lett. 64, 633 (1990). 
10. T. Tanabe, M. Tomizawa, K. Chida, T. Watanabe, S. Watanabe, M. Yoshizawa, H. 
Muto, K. Noda, M. Kanazawa, A. Ando, and A. Noda, Phys. Rev. A45, 276 (1992). 
11. J.A. Tanis, E.M. Bernstein, S. Chantrenne, M.W. Clark, T. Ellison, C.C. Foster, W.G. 
Graham, W.W. Jacobs, J.R. Mowat, T. Rinckel, A. Ross, D. Schneider, M.P. Stockli, 
and N.R. Badnell, Nucl. Instrum. & Methods B56/57, 337 (1991). 
12. R.R. Haar, J.A. Tanis, V.L. Plano, K.E. Zaharakis, W.G. Graham, J.R. Mowat, T. 
Ellison, W.W. Jacobs, and T. Rinckel, Phys. Rev. A47, R3472 (1993). 


Dielectronic Recombination in Li+ Ions

https://scholarworks.iu.edu/dspace/bitstream/2022/533/1/iucf1993-1994_100-102.pdf

Dielectronic Recombination in Li+ Ions

Abstract

Similar works

Full text

Available Versions

IUScholarWorks